Functional compensation for the loss of testis-specific poly(A)-binding protein, PABPC2, during mouse spermatogenesis

Intronless Pabpc6 encodes a testis-specific, cytoplasmic poly(A)-binding protein but is dispensable for spermatogenesis in the mouse†

Besides ubiquitous poly(A)-binding protein, cytoplasmic 1 (PABPC1), testis-specific PABPC2/PABPt (in humans, referred to as PABPC3), and female and male germline-specific PABPC1L/ePAB, have been reported in the mouse testis. Recent in silico analysis additionally identified testis-specific Pabpc6 in the mouse. In this study, we characterized PABPC6 and its mutant mice. PABPC6 was initially detectable in the cytoplasm of pachytene spermatocytes, increased in abundance in round spermatids, and decreased in elongating spermatids. PABPC6 was capable of binding to poly(A) tails of various mRNAs and interacting with translation-associated factors, including EIF4G, PAIP1, and PAIP2. Noteworthy was that PABPC6, unlike PABPC1, was barely associated with translationally active polysomes and enriched in chromatoid bodies of round spermatids. Despite these unique characteristics, neither synthesis of testicular proteins nor spermatogenesis was affected in the mutant mice lacking PABPC6, suggesting that PABPC6 is functionally redundant with other co-existing PABPC proteins during spermatogenesis.

Impaired fertility in 4930590J08Rik mutant male mice is associated with defective sperm energy metabolism

Testis-specifically expressed genes are important for male reproduction according to their unique expression patterns. However, the functions of most of these genes in reproduction are unclear. Here, we showed that mouse 4930590J08Rik was a testis-specifically expressed gene. 4930590J08Rik knockout mice exhibited a delay in the first wave of spermatogenesis and a reduction of cauda epididymal sperm. Furthermore, knockout spermatozoa exhibited defective acrosome reactions and decreased progressive motility, which led to impaired in vivo fertilization. Transcriptome analysis of testes revealed that most of the differentially expressed genes in knockout testes were associated with metabolic processes. 4930590J08Rik knockout sperm exhibited oxidative phosphorylation deficiency and were highly dependent on increased anaerobic glycolysis to compensate for ATP demands. Taken together, the 4930590J08Rik-disrupted mouse partially mimics the phenotypes of human asthenospermia and oligozoospermia, which provides a new model for further understanding the pathogenesis of idiopathic male infertility.

Chemogenetic Depletion of Hypophysiotropic GnRH Neurons Does Not Affect Fertility in Mature Female Zebrafish

The hypophysiotropic gonadotropin-releasing hormone (GnRH) and its neurons are crucial for vertebrate reproduction, primarily in regulating luteinizing hormone (LH) secretion and ovulation. However, in zebrafish, which lack GnRH1, and instead possess GnRH3 as the hypophysiotropic form, GnRH3 gene knockout did not affect reproduction. However, early-stage ablation of all GnRH3 neurons causes infertility in females, implicating GnRH3 neurons, rather than GnRH3 peptides in female reproduction. To determine the role of GnRH3 neurons in the reproduction of adult females, a Tg(gnrh3:Gal4ff; UAS:nfsb-mCherry) line was generated to facilitate a chemogenetic conditional ablation of GnRH3 neurons. Following ablation, there was a reduction of preoptic area GnRH3 neurons by an average of 85.3%, which was associated with reduced pituitary projections and gnrh3 mRNA levels. However, plasma LH levels were unaffected, and the ablated females displayed normal reproductive capacity. There was no correlation between the number of remaining GnRH3 neurons and reproductive performance. Though it is possible that the few remaining GnRH3 neurons can still induce an LH surge, our findings are consistent with the idea that GnRH and its neurons are likely dispensable for LH surge in zebrafish. Altogether, our results resurrected questions regarding the functional homology of the hypophysiotropic GnRH1 and GnRH3 in controlling ovulation.

Revisiting poly(A)-binding proteins: Multifaceted regulators during gametogenesis and early embryogenesis

Post-transcriptional regulation faces a distinctive challenge in gametes. Transcription is limited when the germ cells enter the division phase due to condensed chromatin, while gene expression during gamete maturation, fertilization, and early cleavage depends on existing mRNA post-transcriptional coordination. The dynamics of the 3'-poly(A) tail play crucial roles in defining mRNA fate. The 3'-poly(A) tail is covered with poly(A)-binding proteins (PABPs) that help to mediate mRNA metabolism and recent work has shed light on the number and function of germ cell-specific expressed PABPs. There are two structurally different PABP groups distinguished by their cytoplasmic and nuclear localization. Both lack catalytic activity but are coupled with various roles through their interaction with multifunctional partners during mRNA metabolism. Here, we present a synopsis of PABP function during gametogenesis and early embryogenesis and describe both conventional and current models of the functions and regulation of PABPs, with an emphasis on the physiological significance of how germ cell-specific PABPs potentially affect human fertility.

Toward Development of the Male Pill: A Decade of Potential Non-hormonal Contraceptive Targets

With the continued steep rise of the global human population, and the paucity of safe and practical contraceptive options available to men, the need for development of effective and reversible non-hormonal methods of male fertility control is widely recognized. Currently there are several contraceptive options available to men, however, none of the non-hormonal alternatives have been clinically approved. To advance progress in the development of a safe and reversible contraceptive for men, further identification of novel reproductive tract-specific druggable protein targets is required. Here we provide an overview of genes/proteins identified in the last decade as specific or highly expressed in the male reproductive tract, with deletion phenotypes leading to complete male infertility in mice. These phenotypes include arrest of spermatogenesis and/or spermiogenesis, abnormal spermiation, abnormal spermatid morphology, abnormal sperm motility, azoospermia, globozoospermia, asthenozoospermia, and/or teratozoospermia, which are all desirable outcomes for a novel male contraceptive. We also consider other associated deletion phenotypes that could impact the desirability of a potential contraceptive. We further discuss novel contraceptive targets underscoring promising leads with the objective of presenting data for potential druggability and whether collateral effects may exist from paralogs with close sequence similarity.

An evolutionary approach to recover genes predominantly expressed in the testes of the zebrafish, chicken and mouse

Background

Previously, we have demonstrated that genes involved in ovarian function are highly conserved throughout evolution. In this study, we aimed to document the conservation of genes involved in spermatogenesis from flies to vertebrates and their expression profiles in vertebrates.

Results

We retrieved 379 Drosophila melanogaster genes that are functionally involved in male reproduction according to their mutant phenotypes and listed their vertebrate orthologs. 83% of the fly genes have at least one vertebrate ortholog for a total of 625 mouse orthologs. This conservation percentage is almost twice as high as the 42% rate for the whole fly genome and is similar to that previously found for genes preferentially expressed in ovaries. Of the 625 mouse orthologs, we selected 68 mouse genes of interest, 42 of which exhibited a predominant relative expression in testes and 26 were their paralogs. These 68 mouse genes exhibited 144 and 60 orthologs in chicken and zebrafish, respectively, gathered in 28 groups of paralogs. Almost two thirds of the chicken orthologs and half of the zebrafish orthologs exhibited a relative expression ≥50% in testis. Finally, our focus on functional in silico data demonstrated that most of these genes were involved in the germ cell process, primarily in structure elaboration/maintenance and in acid nucleic metabolism.

Conclusion

Our work confirms that the genes involved in germ cell development are highly conserved across evolution in vertebrates and invertebrates and display a high rate of conservation of preferential testicular expression among vertebrates. Among the genes highlighted in this study, three mouse genes (Lrrc46, Pabpc6 and Pkd2l1) have not previously been described in the testes, neither their zebrafish nor chicken orthologs. The phylogenetic approach developed in this study finally allows considering new testicular genes for further fundamental studies in vertebrates, including model species (mouse and zebrafish).

Electronic supplementary material

The online version of this article (10.1186/s12862-019-1462-8) contains supplementary material, which is available to authorized users.

Potential roles of the poly(A)-binding proteins in translational regulation during spermatogenesis

Spermatogenesis is briefly defined as the production of mature spermatozoa from spermatogonial stem cells at the end of a strictly regulated process. It is well known that, to a large extent, transcriptional activity ceases at mid-spermiogenesis. Several mRNAs transcribed during early stages of spermatogenesis are stored as ribonucleoproteins (RNPs). During the later stages, translational control of these mRNAs is mainly carried out in a time dependent-manner by poly(A)-binding proteins (PABPs) in cooperation with other RNA-binding proteins and translation-related factors. Conserved PABPs specifically bind to poly(A) tails at the 3′ ends of mRNAs to regulate their translational activity in spermatogenic cells. Studies in this field have revealed that PABPs, particularly poly(A)-binding protein cytoplasmic 1 (Pabpc1), Pabpc2, and the embryonic poly(A)-binding protein (Epab), play roles in the translational regulation of mRNAs required at later stages of spermatogenesis. In this review article, we evaluated the spatial and temporal expression patterns and potential functions of these PABPs in spermatogenic cells during spermatogenesis. The probable relationship between alterations in PABP expression and the development of male infertility is also reviewed.

Adenylation by testis-specific cytoplasmic poly(A) polymerase, PAPOLB/TPAP, is essential for spermatogenesis

The testis-specific cytoplasmic poly(A) polymerase PAPOLB/TPAP is essential for spermatogenesis. Although this enzyme is responsible for poly(A) tail extension of a subset of mRNAs in round spermatids, the stability and translational efficiency of these mRNAs are unaffected by the absence of PAPOLB. To clarify the functional importance of this enzyme’s adenylation activity, we produced PAPOLB-null mice expressing a polyadenylation-defective PAPOLB mutant (PAPOLB^D114A), in which the catalytic Asp at residue 114 was mutated to Ala. Introducing PAPOLB^D114A failed to rescue PAPOLB-null phenotypes, such as reduced expression of haploid-specific mRNAs, spermiogenesis arrest, and male infertility. These results suggest that PAPOLB regulates spermatogenesis through its adenylation activity.